This page only applies to Bitcoin and Litecoin.
By default the BitBoxApp offers you up to 4x different "Priority" options (fee levels) on the page where you create your transaction:
If you want to choose custom fee rate, go to the BitBoxApp settings and enable "Enable custom fees".
When you are about to make a transaction, you will have another fee option, called "Custom" available.
If you select this specific option, you can enter the fee in sat/vByte.
To see current fee estimates in sat/vByte BTC mempool is accepting and how often it would clear, take a look at https://mempool.space/.
If you choose a low fee, it might take a long time until your transaction gets confirmed on the blockchain.